Longitudinal orientation of a tubular thermoplastic film

ABSTRACT

The improved method and apparatus for longitudinal orientation of a tubular thermoplastic film as it leaves an annular extrusion die aims at a better control of this orientation. On its travel between the exit orifice ( 21 ) and the draw-down means, the at least partly molten film passes an annular frictional device ( 101 ), and the frictional force set-up hereby is variable in controlled manner. This device is cooled from its interior ( 105 ) in controlled manner by means of a fluid cooling medium. The friction may e be controlled by airlubrication with air pressed through holes ( 123 ) in the frictional device or through microporous metal, ( 102 ) or alternatively by sucidng the film against the frictional device. In a preferred embodiment the extrusion out of the die is peripherical extrusion, and in another preferred embodiment the film contains a blend of at least two compatible or compatibilized polymers, and the main proportion of the orientation takes place while one is predominantly in a crystalline state and the other predominantly in a molten state.

[0001] The invention concerns method and apparatus as stated in thetitle. More specifically a substantial proportion of such orientationtakes place by annular frictional means set up between the circular exitorifice of an annular extrusion die and the draw-down means (rollers,belts or the like) which hauls off the tube from the die, when the filmis in molten or semi-molten state.

[0002] The invention has been conceived with a special view to themanufacture of cross-laminates, i.e. laminates comprising two or morefilms which each are uniaxially oriented or are biaxially oriented withone direction dominating, and are laminated with the (dominating)directions of orientation crossing each other. This can in practice bedone by giving a tubular film a generally uniaxial orientation, cuttingit helically to from a web with biassed orientation, and laminating twoor more such webs with the orientations criss-crossing each other. Therecan also be a generally longitudinally orientated web included in thelaminate.

[0003] Alternatively or supplementarily, the orientation on bias can beachieved in generally molten state by “twisting” the tubular film whileit is hauled off form the extrusion die.

[0004] A survey over the technology concerning cross laminated film isgiven in the inventor's WO-A-93/14928.

[0005] More precisely expressed the method of the invention concerns aprocess of forming a tubular oriented film by extruding a flow of atleast one molten thermoplastic material from a circular extrusion die,in which process the flow having left a circular exit orifice in the dieis cooled and is oriented at least in the longitudinal direction whileit still is at least partly molten, whereby the longitudinal orientationtakes place by a pulling force set up between the exit orifice andmoving draw-down means.

[0006] In this process the still at least partly molten flow on itstravel between the exit orifice and the draw-down means passes and is infrictional contact with an annular device (hereinafter the frictionaldevice), and the frictional force set up by this contact is variable incontrollable manner other than by adjusting the temperature in the flowor the tensions in the flow during its contact with the device.

[0007] A method and an apparatus of this kind is known fromDE-A-4308689. That invention carries out the longitudinal orientationmainly in molten state: and the transverse orientation mainly within the“range of crystallization”, whereby the effect of blowing to obtaintransverse orientation is enhanced. In that respect the technologydeviates from the aim of the present invention, which is to promotelongitudinal orientation. However, in DE-A4308689 there is an annularinsert in the bubble which, necessarily although unintendedly, byfriction against the film contributes to its longitudinal orientation.There is an annular nozzle surrounding this annular insert which blowstowards the tube and the insert. At this stage the tube is in “the rangeof crystallization”. The function of those devices is to separate afirst part of the film—“bubble” from the rest, so that the tube can bestrongly blown by over pressure in the “bubble” when the thermoplasticmaterial has been brought into the “range of crystallization”. At thesame time the pressure in the bubble is kept near to the ambientpressure in the zone where the material is fully molten, so thattransverse stretching here is avoided.

[0008] In addition to strong air-cooling from the outside of the bubblein DE-A-4308689, there is internal air cooling in the bubble upstream ofthe mentioned insert. This will also cool the insert, but there is notdisclosed any means for controlling the temperature of this insert. Thefriction between the extruded tube can probably for a given temperatureof the insert and a given pressure in the bubble downstream of theinsert, be controlled by the amount of air blown towards the tube whilethe latter passes the insert, however the prior art does not mentionanything about such control of friction.

[0009] A patent from about 1975 issued to the Dutch Van Leerorganization or one of its subsidiaries (the inventor has not at thetime of filing this application been able to identify it further)concerns longitudinal orientation of the extruded tube in solid stateover a mandrel inside th tube, while the latter is hauled off from theextrusion die.

[0010] However in practice it is very difficult to carry out this methoddue to strong contraction forces which are set up when the solid film isdrawn, and which tries to hold the tube firmly to the mandrel.

[0011] Finally it should be mentioned that mandrels inside the extrudedtube have been widely used for calibration of the tube. As examplesreference is made to GB-A-2112703 and to EP-A-0285368.

[0012] The present invention being the process of forming a tubularoriented film and the device therefor is characterised by the frictionaldevice, which either can be arranged inside the bubble or outside thebubble being cooled from within by means of a fluid cooling medium togive its surface in contact with the flow a controllable temperature,and further that this temperature and the said friction is controllableto produce between the said frictional device and the draw-down means, acontribution to the longitudinal orientation which makes the totallongitudinal orientation whereby the tubular product film has alongitudinal shrinkability. The shrinkability preferably is of a factorof no less than about 4, referring to shrink testing carried out at theupper limit of the melting range of the extruded film, that is the film,when heated to the shrink testing temperature shrinks in thelongitudinal direction to one quarter or less of its length.

[0013] Having left the frictional device the tubular film may be allowedto contract during the longitudinal stretching, or the air pressureinside the bubble may maintain the diameter of the tube or even lead thetube to become strongly blown to obtain transverse orientation. Suchblowing normally requires special precautions to be taken, which shallbe mentioned later.

[0014] By use of the present invention, as this is defined above, thelongitudinal melt-orientation can be adjusted with a particularprecision and/or be made particularly strong. This has importance forseveral uses especially for the above-mentioned use in cross-laminates.

[0015] For achieving a particular high melt orientation, an embodimentof the invention is characterised in that the main proportion of theorientation takes place while the polymer material or materials partlyis/are molten and partly crystallized. Preferably at least 5% of thepolymer material or materials should be crystallized during thatorientation. Thus the polymer flow may advantageously contain a blend ofat least two compatible or compatibilised polymers, and the mainproportion of the orientation then should take place while one polymeris predominantly in a crystalline state and the other is predominantlyin a molten state.

[0016] Another embodiment of the invention is characterised in that thefriction between the frictional device and the film is controlled byair-lubrication with air which is pressed through holes in thefrictional device or through microporous metal, which forms at least azone of the surface which the flow contacts.

[0017] Alternatively, the friction may be controlled by sucking the flowagainst the frictional device. Thus the suction can be applied throughmicroporous metal, or the surface which the flow contacts can have agrooved pattern, whereby the grooves are circular around the die axis.The grooves are then subjected to a controlled under pressure.

[0018] The pulling force on the frictional device can be monitored andused through feed-back means for adjustment of the over- orunder-pressure which determines the friction, whereby the degree oforientation is controlled.

[0019] In case the extruded tubular film is particularly thick and/orfrom a polymer of a particularly high molecular weight, the frictionaldevice may have a surface temperature in or above the melting range ofthe main body of the film. However, this an exception, and normally thisdevice should have a temperature which, when the film is coextruded andhas a low melting surface layer on the side facing this device, is evenlower than the melting range of this surface layer, otherwise it may betoo difficult to obtain a frictional but smooth gliding of the film overthe frictional device. This means that the time of contract must be soshort that only a very thin surface layer will solidify, while the mainbody of the film maintains a temperature which is near the predeterminedtemperature of stretching. The thin solidified layer will melt or partmelt again when it has left the frictional device by heat from theinterior of the film.

[0020] In order to achieve a particular high frictionally controlledorientation, the temperature of the film, during the stretching must bekept within the crystallisation range or slightly above this, as italready appears from the foregoing. Under such circumstances the filmshould normally be efficiently cooled before it meet the frictionaldevice. For this purpose an embodiment of the invention is characterisedin that upstream of the frictional device there is a generally annular,cylindrical or conical part (hereinafter the shock-cooling part)installed for cooling inside or outside the bubble. The flow passes andcontacts this in a generally frictionless or low friction manner asestablished e.g. by air lubrication through microporous metal or throughholes. This part is cooled from its inside by means of a fluid coolingmedium and kept at a temperature which is sufficiently low to take awayat least half of the heat needed to bring the temperature in the flowdown to the desired value for the orientation.

[0021] Upstream of the frictional device but downstream of the justmentioned shock-cooling part if such part is used, there is a part(hereinafter the “temperature fine adjustment part”) of a similarconstruction as the shock-cooling part, but adapted for a fineadjustment of the average temperature in the flow.

[0022] The following succession of apparatus parts are preferably inclose proximity to one another or mutually connected throughlow-heat-transfer connections:

[0023] a) the diepart forming one side of the exit orifice,

[0024] b) the shock-cooling part if present;

[0025] c) the temperature fine adjustment part if present,

[0026] d) the frictional device.

[0027] All apparatus parts in this succession are on the same side ofthe bubble, inside or outside.

[0028] A preferable way to achieve even and efficient cooling of thetubular film immediately upon its exit from the die, is for the flow toleave the exit orifice under an angle of at least 20° to the axis of thedie, its direction of movement pointing either away from or towards theaxis, and then meet a cooling part which is in close proximity to theexit orifice or connected to the diepart forming one side of the exitorifice. (Of course the channel forming the exit orifice then shouldalso form an angle of close to 20° or more to the die axis). Thementioned cooling part will be the shock-cooling part described above ifthat is used, or otherwise the temperature fine adjustment part alsodescribed above if that is used or otherwise the frictional device. Asubstantial part of the zone in which the flow follows the mentionedpart should be rounded when seen in axial section, so that the filmgradually is turned at least 20° in the direction towards the die axiswhile it moves over this part or this assemblage of parts.

[0029] This arrangement of the exit from the die can be achieved veryconveniently when the exit orifice of the die either is at the outerperipherical surface of the die or, if the die has a central cavitywhich is defined by an inner periphery, is at the inner periphericalsurface of the die. This is also a very practical arrangement inconnection with the start-up of the extrusion since it then is easier toget hold of the molten mass and feed it over the cooling and temperaturecontrolling annular parts.

[0030] Thus it is advantageous for the flow to leave the die under anangle of 90° or close to 90° to the axis. This has the additionaladvantage that the gap of the exit orifice can be adjusted from locationto location as is usually with flat dies. To achieve this at least oneside of the exit orifice can be defined by a lip which is sufficientlyflexible to allow different adjustments of the gap of the orifice fromlocation to location. Simple mechanical devices like push-pull screws ormore sophisticated devices, known from construction of flat dies, can beused for this.

[0031] It is noted that peripherical extrusion as such is known, seeU.S. Pat. No. 2,769,200 (Longstretch et al.), U.S. Pat. No. 2,952,872(Buteau et al.), U.S. Pat. No. 3,079,636 (Aykanian) and U.S. Pat. No.3,513,504 (Ronden et al). The purpose of peripherical extrusion in thesepatent is to achieve a high blow ratio without any damage to the film.These patents do not disclose the use of an annular device to turn thedirection in which the film moves from the transverse toward a moreaxial direction, but they do disclose the adjustment means at the exitorifice.

[0032] As mentioned in the foregoing the tubular film may be allowed tocontract circumferentially during the longitudinal stretching, while itis hauled off from the frictional device—in this way the orientation maybecome truly uniaxial—or it may, by an inside pressure, maintain itsdiameter or even become blown by a relatively high over-pressure andthereby achieve a significant transverse orientation in addition to thelongitudinal components of orientation. The following measures can betaken in order to avoid the over-pressure acting on the tubular filmbefore the latter leaves the frictional device:

[0033] If the frictional device is inside the bubble, the part of theair which is contained in the flow before the latter meets thefrictional device (hereinafter air 1) is closed off from the air whichis contained in the flow after the latter has left the frictional device(hereinafter air 2), and air 2 is kept under a pressure which issubstantially higher than the pressure in the ambient atmosphere, whilethe pressure in air 1 approximately is kept at this ambient pressure. Ifthe frictional device is outside the bubble, there is provided a closedspace between the die and the frictional device for the air surroundingthe bubble, and the air pressure inside the bubble is kept substantiallyhigher than the ambient pressure, while the outside pressure within theclosed space approximately matches the pressure inside the bubble.

[0034] As it has been emphasized in the foregoing a particularlyimportant application of the invention is in the manufacture ofcross-laminates. For this and several other uses, the flow leaving thedie should normally be a coextrudate of two, three or more layers, e.g.a main layer for tensile strength in the middle and thin laminationand/or heatseal layers on one or both surfaces. For the manufacture ofcross-laminates the parameters of the process should be adapted toprovide a tubular film with an orientation which is predominantlylongitudinal or follows a helical direction in the tube. To obtain apredominantly helical or “screwed” orientation there can be establisheda rotation between a first end comprising the draw-down means and asecond end comprising the extrusion die with the frictional device, theshock-cooling part (if this part is used), and the “temperature fineadjustment part” (if this part is used).

[0035] The invention shall now be explained in further detail withreference to the drawings, which all show sections made through the axisof the annular extrusion die.

[0036]FIG. 1 shows the last part of a coextrusion die with connectedfrictional device over which the film is bent during the haul off. Theextrusion is outwardly peripherical through an exit orifice in theexternal periphery of the die.

[0037]FIG. 2 is similar to FIG. 1, but showing inwardly periphericalextrusion through an exit orifice in the internal periphery of the die,which has a wide tubular cavity around its centre.

[0038]FIG. 3 is similar to FIG. 1, but in addition to the frictionaldevice there is a shock-cooling-part and atemperature-fine-adjustment-part.

[0039]FIG. 4 is similar to FIG. 3, but for inwardly periphericalextrusion like in FIG. 2.

[0040]FIG. 5 shows the last part of a coextrusion die in which the exitorifice is arranged through the generally plane die surface, which isperpendicular to the axis, like an annular die for film extrusion, butwith the exit orifice pointing inwardly under an angle of about 20°. Thedie is supplied with a shock-cooling-part, atemperature-fine-adjustment-part, and a frictional device.

[0041]FIG. 6 shows, in about natural size, a modification of the“frictional device” of FIG. 3.

[0042] The peripherical annular coextrusion of which the outward part isshown in FIG. 1, can conveniently be the die which in full is shown inthe inventor's copending WO-A-02/51617 FIGS. 7 to 9 (one of the patentapplications, from which priority is claimed for the present case). Thereference numerals for the die itself are also taken from these figures.The die axis is parallel with the dot-dash line (1), but as the arrowindicates the real axis is much more to the right in the drawing. Otherconstruction of the peripherical coextrusion die can be of course alsobe used.

[0043] The die is assembled from bowl and disc-formed parts, of which(5), (6), (7 a) and (7 b) appear from FIG. 1. Three components (A), (B)and (C) are coextruded to form the film B/A/C. If the invention is usedto make films to become cross-laminated, (A) which forms the middlelayers would be the layer to supply the main strength, while (B) and(C), the surface layers, should form lamination and/or heatseal layers(referring to the above mentioned patents regarding cross-laminationtechnology). They should then exhibit lower melt ranges and normallyalso lower melt viscosities than (A). As a practical example, (A) may bea compatibilised blend of 25% homo-polypropylene of a relatively highmolecular weight, 25% HMWHDPE and 50% LLDPE, (C) if chosen as heatseallayer can be plain LLDPE, and (B) if chosen as lamination layer can be alow melting copolymer of ethylene as e.g. EPDM or low meltingmetallocene polyethylene—or a blend or such polymer with LLDPE, (B)merges with (A) at the internal orifice (19) while (C) merges with (A)at the internal orifice (20). These two orifices are here shownimmediately adjacent to each other, and for rheological reasons this isvery advantageous when the surface components have lower meltviscosities than the middle component.

[0044] The three merged components proceed through the exit channel (18)towards the exit (21) in radial direction. Having left the exit, thetubular B/A/C-film is pulled, still in a radial direction, towards theouter surface of “the frictional device” (101). Here it is bent upward,following the surface of the “frictional device” (101), which forms partof a toroid (“donut-shape”). During this travel it is cooled by thefrictional device (101) and is air lubricated, but in a controlledmanner so that there is a controlled friction between the frictionaldevice (101) and the film. The friction in combination with thetemperatures in the B/A/C film controls the longitudinal orientationwhich is introduced in the film. The means for air lubrication,temperature control and control of friction are explained below.

[0045] Having left the frictional device (101), the B/A/B film may, byan over-pressure within the bubble, have its diameter expanded andthereby also get a significant transverse orientation, but if asignificantly uniaxial character of the orientation is preferred, theblow ratio should be very low or may even be inverse (contraction). Dueto rather high contracting forces during the longitudinal stretchingthere should normally be established an over-pressure inside the bubblealso where the tube contracts.

[0046] Having left the frictional device (101) the B/A/C-film is furthercooled by air (not shown), preferably both external and internalcooling, in well-known manner. It is hauled off also in well-knownmanner (not shown) by use of a collapsing frame and driven rollers, andnormally thereafter spooled up as a flat film. Due to high stretchingforces it may be necessary to substitute the collapsing frame by a setof converging transport belts, a method which also is known, e.g. fromthe above mentioned U.S. Pat. No. 3,513,504.

[0047] At the exit orifice (21) one dielip (25) is made adjustable withthe possibility to have the gap varying around the circumference andthereby compensate for accidental differences in the flow. This can bedone in simple manner when the channel here is flat (as shown) or almostflat instead of being pronouncedly conical or being tubular. Theadjustment can be made by a circular row of screws, of which one (26) isshown. It is sketched as a simple screw but could also be a push-pullscrew. Instead of screws there can also e.g. be used thermally expansivedevices as now used for similar adjustments of the exit orifice in flatdies.

[0048] As already mentioned it is not new to carry out periphericalextrusion, and in this connection such adjustment of the exit orifice isalso known. However, it is of particular importance in connection withthe present invention, since the normal precautions to achieve even filmthickness would cause difficulties. These normal precautions work on theprinciple of different cooling of the extruded tubular film at differentcircumferential positions, either established by local air cooling ofthe bubble, or differential local cooling of a dielip. However, suchsystems do not combine well with the contact cooling of the film used inthe present invention.

[0049] Details regarding the air lubrication and the cooling of theB/A/C-film on the frictional device (101), and means to controlsfriction and temperature, will now be explained. The frictional device(101) can be made of steel, and almost the whole of the surface whichthe film passes over, is made from microporous metal, shown as a roundedplate (102). This can be is screwed to the base steel part of thefrictional device (101). (None of the drawings will show any of thescrews used to connect the different dieparts). The microporous platecan conveniently have pore size around 0.01 mm. The compressed air forthe air lubrication is fed through a number of pipes, of which FIG. 1shows one (103). It is distributed over a network of channels in (101).The drawing shows only the channels (104) which extend circularlycentred on the axis of the die. The drawing does not show the channelswhich extend perpendicularly to channels (104). In some cases thereshould be applied suction instead of over-pressure, namely when the filmis especially thick and/or of an especially high average molecularweight.

[0050] The frictional device (101) is supplied with an annular cavity(105) for circulation of a cooling fluid, e.g. water, oil or air. Thecirculating fluid allows the temperature of the surface of (101) to becontrolled within a few degrees. For that purpose there can be provideda thermocouple relatively close to the surface (not shown).

[0051] The cooling fluid is directed in and out of the annular cavity(105) through pipes of which one (106) is shown. These pipes and theother pipes mentioned above and below pass out through a large cavity atthe centre of the die, which cavity appears from the above mentionedFIG. 7 in patent application WO-A-0251617. The pipes for the coolingfluid are connected with a circulation pump and a heating/cooling unit.Similarly, the above mentioned pipes (103) are connected with anair-accumulator and a compressor (or vacuum pump if suction is used)with means to adjust the pressure.

[0052] The frictional device (101) is fixed to diepart (6) through anumber of arms (e.g. three or four) of which one (107) is shown. Diepart(6) has corresponding arms (108) each of which is fixed to an arm (107)through a heat insulating plate (109). This is done in order to avoidany significant heat transfer between the hot diepart and the muchcolder frictional device. Each of the arms (107) has a relatively thinbridge part (110), thin enough to achieve measurable variations inbending with variable pull in the film, and at least one of these thinportions is supplied with a suitable dynamometer e.g. a strain gaugedevice (111). Signals from this device are fed to the devices whichcontrol the over-pressure or vacuum, reducing or increasing the frictionbetween the film and the frictional device (101), so that theorientation is kept at the desired value. In order not to make too muchresistance against the bending of (111), each of the pipes (103), (106)and (112)—the last will be described below—may comprise a corrugatedsegment (not shown).

[0053] Internal air cooling and the air pressure required to maintainthe blow ratio which has been set, are established by conventionaldevices. The devices pass through the above mentioned cavity at thecentre of the die. This is closed off from the environment. A thin plate(113), installed between diepart (6) and frictional device (101)separates the inside of the bubble, which is held under pressure, fromthe space (114) between die and frictional device, and this space iskept at about ambient pressure through the pipe (112). Without thedividing plate (113) the film would be ruined by the pressure inside thebubble as it leaves the exit (21).

[0054] Since, roughly speaking, about half of the heat used to cool downthe film to about ambient temperature, will be taken by the contactcooling, and normal air cooling systems used thereafter, the “tower”with haul-off devices can be very short. If a helically extendingorientation is wanted, these haul-off devices may rotate around the dieaxis, and the flat tubular film may be reeled up at the top of the“tower”.

[0055] Using the above mentioned example in which the main layerconsists of a blend of homo-PP (solidifying at about 160° C.), HMWHDPE(solidifying at about 125° C.) and LLDPE (solidifying at about 120° C.),the film will leave the exit (21 ) with a temperature of about 220-240°C. and to achieve a convenient high longitudinal orientation, aconsiderable amount of the draw down can e.g. take place between130-150° C. To achieve sufficiently quick cooling, and also to avoidthat the lower melting surface layer inside the bubble sticks to (101),the latter may be cooled e.g. to about 50° C. The length of thefilm-travel over the surface of (101) must be adapted so that, when thefilm leaves (101), its average temperature still will not have reacheddown to 125° C. A thin part of the film directly contacting (101) willbe cooled below this and solidify, but will melt again when, the filmhas left (101).

[0056] Depending on the balance between longitudinal draw-down ratio,temperatures and frictional resistance, the majority of this draw-downmay take place before or after PP has crystallized. Thus e.g. a 2.5 mmthick film leaving exit (21) may be drawn down to a thickness of 0.250mm before the PP solidifies and thereafter drawn down to a thickness of0.05 mm.

[0057] In FIG. 2, relating to extrusion out of a peripherical exitleading into an interior cavity in the circular die, the die axis isindicated by the dot-and-dash line (1). The upper part of this cavity isclosed off from the atmosphere by means of the circular plate (115).Over this plate, inside the bubble there is kept an over-pressure, andthere is internal cooling. Devices for imposing the pressure and coolingare not shown. By means of the thin plate (113) the space (114) isseparated from the atmosphere, and the pressure in this space is throughthe pipe (112) kept at approximately the same value as the pressureinside the bubble (which is shown on the left of the film). In otherrespects FIG. 2 can fully be understood on basis of what is explained inconnection with FIG. 1.

[0058] It appears from the description of FIG. 1 that it is relativelydifficult to obtain the most desirable combination of orientation anddraw-down ratios with the relatively simple devices shown in FIGS. 1 and2. The more complicated devices shown in FIGS. 3 to 5 improve theserelations. In each construction there are used three independent parts:

[0059] a) a shock-cooling part (116),

[0060] b) a temperature-fine-adjustment part (117) and

[0061] c) a frictional device (118).

[0062] The three parts are kept thermally insulated from each other byinsulating plates (119). Each of the three parts have devices fordirecting air for lubrication—or in the case of (118) it may be forsuction—and for circulation of a cooling/heating fluid, which aresimilar to those devices as explained in connection with FIG. 1. Thethree parts are controlled independently of each other. During thepassage over parts (116) and (117) the friction is controlled usinginformation from the strain gauge device (111). The dotted lines (120)show grooves through which the compressed air used for air lubricationcan escape.

[0063] As is explained in connection with FIGS. 1 and 2, it is importantto avoid any significant pressure difference between the two sides ofthe film when the latter leaves exit (21). This is achieved by the useof separation walls (121 and 122).

[0064] In the variation of the frictional device (118) shown in FIG. 6the friction is controlled by suction, but not through microporousmetal. Instead of this there are grooves (124) in this part, e.g. with apitch of about 3 mm and about 2 mm deep and 1 mm wide, with roundedcrests (125), and a controlled vacuum is applied through the holes(123).

[0065] Using again the afore-mentioned example of suitable materials,the shock-cooling part (116) can conveniently be kept at a temperaturewhich cools the film to about 140-150° C., thetemperature-fine-adjustment part (117) at a temperature so as to adjustthis temperature of the film more exactly e.g. at 145° C., and part(118) can be kept at 50° C. to avoid sticking. The passage over thefrictional device (118) takes so short time that the drop in averagefilm temperature will be very low.

1. In a process of forming a tubular oriented film of at least onethermoplastic polymer material having a crystallisation range byextruding a flow of at least one molten thermoplastic polymer materialfrom a circular extrusion die in the form of a continuous tubularbubble, in which process said flow having left a a circular exit orificein the die is cooled and is oriented at least in the longitudinaldirection while still being at least partly molten, said longitudinalorientation taking place by a pulling force set up between the exitorifice and moving draw-down means, and in which process the still atleast partly molten flow on its travel between the exit orifice and thedraw-down means passes and is in frictional contact with a surface of anannular friction device, the improvement of cooling the frictionaldevice, which can be arranged inside or outside said tubular bubble,from within by means of a fluid cooling medium to give said surface incontact with the flow a controllable temperature, the frictional forceset up by the frictional contact being variable in controllable mannerother than by adjusting the temperatures in the flow or the tensions inthe flow during its contact with this device, the temperature of thesurface of the frictional device and the friction being adapted toproduce, between the frictional device and the draw-down means, acontribution to the longitudinal orientation, while the temperature inthe flow is within the said crystallisation range or slightly abovethis, whereby the tubular film product has longitudinal shrinkability.2. A process according to claim 1 in which the longitudinalshrinkability is of a factor of no less than about 4, as determined byshrink testing carried out at the upper limit of the melting range ofthe extruded film.
 3. A process according to claim 2, wherein theorientation takes place mainly while such polymer material partly ismolten and partly crystallised.
 4. A process according to claim 3,wherein at least 5% of such polymer material is crystallised.
 5. Aprocess according to claim 1 wherein the polymer flow contains a blendof at least two compatible or compatibilised polymers, and the mainproportion of the orientation takes place while one is predominantly ina crystalline state and the other is predominantly in a molten state. 6.A process according to claim 1 including the step of controlling thefriction by air lubrication with air passed through holes or fines poresin at least a portion of the surface of the frictional device which iscontacted by the polymer flow.
 7. A process according to claim 1,wherein the friction is controlled by sucking the polymer flow againstsaid surface of the frictional device.
 8. A process according to claim7, wherein said friction is applied through an array of micropores insaid surface.
 9. A process according to claim 7, wherein the surfacecontacted by which the polymer flow has a grooved pattern, the groovesbeing circular around the die axis, and the grooves are subjected to acontrolled negative pressure.
 10. A process according to claim 1including the steps of monitoring the tension created in the polymerflow by the frictional device and adjusting the level of such frictionalforce in response to the results of such monitoring, whereby the degreeof orientation is controlled.
 11. A process according to claim 1 whereinupstream of the frictional device there is a generally annular,cylindrical or conical shock-cooling member inside or outside saidbubble over which the tubular polymer flow passes in a generallyfrictionless or low friction manner, said member being cooled from itsinside by means of a fluid cooling medium and kept at a temperaturewhich is sufficiently low to remove from the flow at least helf of theheat needed to bring its temperature to the desired value for theorientation.
 12. A process according to claim 11 in which the polymerflow past the shock-cooling device is lubricated by air lubricationthrough micropores or through holes.
 13. A process according to claim 11which includes the step of passing the tubular polymer flow upstream ofthe frictional device, and downstream of the shock-cooling member insubstantially frictionless manner over a fine temperature adjustingmember which is annular, conical or cylindrical and is cooled from itsinside by a fluid cooling medium for a fine adjustment of the averagetemperature in the flow.
 14. (Cancel.)
 15. (Cancel.)
 16. (Cancel.)
 17. Aprocess according to claim wherein the exit angle of the polymer flowfrom the die orifice is substantially 90° to the die axis.
 18. (Cancel.)19. A process according to claim 1 wherein at least two layers of saidmolten thermoplastic polymer material is coextruded from said circulardie orifice to form said polymer flow.
 20. A process according to claim1 wherein said frictional device is associated with said circularextrusion die for rotation therewith and including the step of rotatingone of said extrusion die and said draw-down means relative to the otherabout the axis of the tubular film flow in order to produce a tubularfilm with a predominately helical orientation.
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 25. In an apparatus for extruding moltenthermoplastic polymer material comprising an annular die having acircular exit orifice through which the molten material is extruded as atubular flow, downstream of said exit orifice moving draw-down means forapplying longitudinal tension to the extruded tube whereby the materialof the tube is oriented in the longitudinal direction while still atleast partially molten, and, disposed between the exit orifice and thedraw-down means, an annular frictional device having an annular surfacearranged for frictional contact with either the inside or outsidesurface of the tubular flow, the improvement wherein the frictionaldevice comprises tension-controlling means to allow control of thecoefficient of friction between the surface of the tubular flow andannular surface of the device and said device has an interior cavity andmeans for flowing through said cavity a fluid cooling medium to coolsaid annular surface, whereby the tension in the tubular polymeric flowbetween the draw-down means and the frictional device may be varied tocontrol the longitudinal orientation of said flow.
 26. Apparatusaccording to claim 25 in which the annular surface of the frictionaldevice in contact with the surface of the tubular flow is provided withholes or micropores passage of air therethrough.
 27. Apparatus accordingto claim 26 further comprising a vacuum source connected to said holesor micropores on the opposite side of said surface from the tubular flowin contact therewith for imposing a negative air pressure therein. 28.Apparatus according to claim 25 in which the annular surface of thefrictional device has a pattern of grooves therein which are circulararound the die axis and including means for connecting said grooves to asource of negative air pressure.
 29. Apparatus according to claim 25which further comprises means for measuring the force exerted on thefrictional device by the tension in the tubular polymeric flow passingthereover and means responsive to such measurement to control thecoefficient of friction between the flow surface and the annular surfaceof the friction device.
 30. Apparatus according to claim 25 furthercomprising a shock-cooling menber of generally annular, circular,cylindrical or conical configuration disposed upstream of the frictiondevice, said member having a surface over which the extruded flow passesin a substantially frictionless contact, said member being cooled by aflow of cooling medium through its interior whereby at least half theheat required to cool the tubular flow to a temperature suitable fororientation may be removed from the flow.
 31. Apparatus according toclaim 30 in which the shock-cooling member is provided with airlubrication means to lubricate the passage of the tubular flow over thesurface thereof.
 32. Apparatus according to claim 31 in which the airlubrication means comprises holes or micropores in the surface of theshock-cooling member and a source of positive air pressure connected tosaid holes or micropores on the side thereof opposite said tubular flowfor imposing a flow of air through the holes or micropores against thetubular flow.
 33. Apparatus according to claim 30 further comprising agenerally annular fine temperature adjustment member of circular,cylindrical or conical configuration upstream of the friction device anddownstream of the shock-cooling member, over which the extruded tubularflow passes in a substantially frictionless manner, said finetemperature adjustment member being is cooled by a flow of coolingmedium through its interior.
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 35. Apparatus according toclaim 25 wherein the die is arranged for the flow of extruded materialto leave the exit orifice at an angle of at least 20° to the axis of thedie, the angle being directed towards or away from the axis, and saidfrictional device has a rounded profile when seen in axial sectionwhereby the tubular flow of material passes around said rounded profileto undergo a change in direction of at least 20°.
 36. Apparatusaccording to claim 35 in which the exit orifice is at the outerperipheral surface of the die.
 37. Apparatus according to claim 36 inwhich the annular frictional device extends around the inside of thebubble.
 38. Apparatus according to claim 35 in which the exit orifice isat the inner peripheral surface of the die.
 39. Apparatus according toclaim 38 in which the annular frictional device extends around theoutside of the bubble.
 40. Apparatus according to claim 36 in which thetubular flow of extruded material leaves the die at an angle of about90° to the axis.
 41. Apparatus according to claim 37 wherein thefrictional device is disposed in the interior of the tubular bubbleproximate to the die orifice and which comprises enclosure means forisolating the space in the interior of the tubular bubble upstream ofthe frictional device from the space in the interior of the tubularbubble downstream of the frictional device and means for imposing ahigher air pressure in the space downstream of the frictional devicethan in the space upstream thereof.
 42. Apparatus according to claim 39wherein the frictional device is disposed on the exterior of the tubularbubble proximate to the die orifice and which comprises enclosure meansfor isolating from the ambient atmosphere the space between the die andthe frictional device and for imposing an air pressure higher thanambient in the space thus isolated and means for closing the space inthe bubble from the atmosphere and for imposing an air pressure higherthan ambient in the bubble.
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 44. Apparatus according toclaim 25 in which the draw-down means is mounted for rotation relativeto the extrusion die and said frictional device.
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 46. Aprocess according to claim 1 wherein the frictional device is disposedinside the bubble proximate to the die orifice to create a space withinthe bubble between the frictional device and the die orifice and thespace thus created is isolated from the interior of the bubbledownstream of the frictional device, and including the step ofmaintaining within the interior of the bubble downstream of thefrictional device an air pressure which is substantially higher thanambient air pressure while the air pressure within the isolated space ismaintained at substantially ambient pressure.
 40. A process according toclaim 1 wherein the annular frictional device is disposed around theenterior of the bubble proximate to the die orifice to create an annularspace therebetween and the thus created annular space is isolated fromthe ambient atmosphere and including the step of maintaining within theinterior of the bubble an air pressure that is substantially higher thanatmospheric pressure while the air pressure within the isolated space isheld approximately equal to the pressure within the bubble.